1. Field of the Invention
The present invention relates to a mobile communications control method and system, and more particularly to a mobile communications control including a transmit power control and information transmission bit rate control on a radio communications channel formed among at least one a base station and at least one mobile station in mobile communications that are typically portable telephones.
2. Description of the Related Art
Mobile communications employ a transmit power control to reduce the transmit power to the minimum level at which a given communication quality can be satisfied. The transmit power control makes it possible to reduce interference with a communication involved in another mobile station and thus improve the communications quality and to increase the capacity of the entire system. It is also possible to reduce consumption power and save the battery installed in the mobile station.
Particularly, in a radio access system of CDMA (Code Division Multiple Access), the transmit power control is essential to decrease interference as low as possible because an increase in the subscriber capacity directly depends on the decrease of interference. In CDMA, there is a proposal in which the transmit power of the mobile station is controlled so that the received CIR (Carrier Interference Ratio) in the base station becomes equal to a given target CIR, while the transmit power of the base station is controlled so that the received CIR in the mobile station becomes equal to a given target CIR. Here, the CIR is a ratio between a desired signal and an interfering signal and is computed by dividing a desired-signal power obtained by despreading, using a spreading code, of a received signal including a signal spread by the above spreading code on a transmitting side by an interfering-signal power.
FIG. 1 shows an example of a method of controlling the transmit power of the mobile station. Referring to FIG. 1, when the received CIR in a base station 1001 is lower than the target CIR, the base station 1001 transmits a transmit power control signal “1” in order to increase the transmit power of a mobile station. A mobile station 1002 receives the transmit power control signal “1”, and increases the transmit power by, for example, 1 dB. In contrast, when the received CIR in the base station 1001 is higher than the target CIR, the base station 1001 transmits a transmit power control signal “0” in order to decrease the transmit power of the mobile station. The mobile station 1002 receives the transmit power control signal “0”, and decreases the transmit power by, for example, 1 dB.
In CDMA, when an increased number of mobile stations simultaneously communicates within an identical cell, interference increases and an increased transmit power is needed to satisfy the target CIR. A limited transmit power is available due to the characteristics of a transmission amplifier. Thus, if the number of mobile stations that simultaneously communicate exceeds a certain number, a mobile station will not be able to increase the transmit power to the target CIR.
Conventionally, a call admission control is carried out so that the number of mobile stations that are allowed to simultaneously communicate falls within a limit placed upon capacity. Thus, the average number of mobile stations that are in communications falls within the capacity limit. Therefore, it is possible, in circuit switched communications, to decrease the frequency of occurrence of a situation in which a call in communications is disconnected within a predetermined threshold and, in packet communications, to prevent occurrence of a situation in which packets are not transmitted so that the throughput greatly decreases and sometimes information cannot be transmitted at all.
However, in the mobile communications, the power of interference with a particular mobile station and the magnitude of the desired signal power change momentarily due to movement of the mobile station and a fluctuation of the received signal level, namely, fading. Even if all mobile stations satisfy the given quality at the time of call admission and channel assignment in handover, the given CIR and given communication quality may not be obtained due to an increase in interference and reduction in receiving desired power resulting from movement of mobile stations.
As described before, there is a limit on the transmit power. A mobile station that is located close to the base station may control the transmit power within the limit thereon. However, a mobile station that is located away from the base station may not anytime satisfy the given communications quality even at the maximum transmit power. In such a case, a call which utilizes a radio channel that does not satisfy the given communication quality is forced to be disconnected. The case mentioned above will frequency occur in a situation in which a sufficient transmit power is not available and the traffic increases.
Even when the traffic is not high as compared to the system capacity, the given communications quality may not be available at the maximum transmit power in a case where the mobile station is located away from the base station, in a cell periphery or in another case where the mobile station is located in the interior of a building and the received level is not high. In such cases, the call using the radio channel that cannot satisfy the given communications quality is forced to be disconnected. In packet communications, the target CIR may not be obtained and the packet loss rate may increase, so that the throughput may greatly be reduced.
As described above, the call which does not satisfy the given communications quality is disconnected. Thus, the conventional technique has a high ratio of call disconnect during communications and a low grade of service. If the number of simultaneous communications channels is reduced in order to increase the rate of call disconnect during communications, the system capacity will reduce. In packet communications, the throughput is reduced and the delay increases.